Because when the Doctor comes to take you away as his latest companion, you'll need good core strength. For all the climbing, hanging-on, and running you'll obviously be doing.
Anytime someone runs
"I am the Doctor" by Murray Gold plays
The Doctor says "I'm sorry," or describes something as "cool"
You see an additional species of alien (other than the Doctor)
You hear the TARDIS
The Doctor explains the historical significance of a place or event
.....10 leg lifts
Someone wears a silly hat, glasses, or a period costume
Something "can't be" or is "impossible"
.....10 reverse crunches
Additional companion is featured in the episode
A well-known villain is featured (Angels, Daleks, Cybermen, etc.)
You see the sonic screwdriver or psychic paper
.....30 second side plank each side
They split up the group, or the companion and Doctor are split up
Anytime the Doctor gets angry and shouty
.....20 oblique/side crunches each side
Anyone says "Doctor!" or he says "I'm the Doctor"
Any time someone (including the Doctor) describes him as smart, clever, brilliant, etc.
.....20 Russian twists